Practical and effective light delivery during behavioral modulation is a key challenge in applying optogenetics to understand and control neural function. Initial solutions to this problem have relied on tethered optical fiber-based systems, in which a fiber optic is inserted into the brain of an animal. Such systems exploit the stable nature of the brain-skull interface, enabling persistent optogenetic modulation of identified neural populations. These systems have been refined over the past decade, such as by allowing fiber rotation during animal movements using optical and electrical commutators and by improving the ease of attachment and detachment. These tethered systems nonetheless impose significant constraints on experimental design and interpretation, both by requiring investigators to handle and physically restrain animals to attach an optical fiber prior to behavioral testing, and by limiting the environments in which optogenetic experiments can be performed.
Recent efforts have been made to eliminate tethers by delivering light via wireless headmounted systems. In one design, a battery-powered, wirelessly-controlled device that delivers light through a thinned mouse skull with an LED. That system has subsequently been improved by decreasing its size, and was used to demonstrate motor activation using optogenetic stimulation of primary motor cortex. Another solution advanced wireless optogenetics through the use of a wirelessly powered system that removed the need for bulky batteries. Another approach developed a flexible, injectable LED-based system for optogenetic stimulation that was capable of stimulating deeper brain regions and could be powered by a head-mountable wireless power receiver.
These advances of wireless optogenetic technology, although trailblazing, have been limited by the mass and size of the devices. The reported wireless systems weigh 0.7 to 3 g (the mass of a mouse head is approximately 2 g). While the smallest wirelessly powered device weighs 0.7 g, it lacks remote-control. All previous devices are so large that they protrude several millimeters beyond the skin and cannot be left attached to the animal for prolonged periods of time. Head-mountable devices of this mass and size ultimately limit which central nervous structures can be targeted, and prohibit optogenetic control of the spinal cord or peripheral nervous system. Further, they hinder the animal's freedom of movement and behavior by preventing animals from entering small enclosures or engaging in normal social interactions with other mice.
No fully internal device has yet enabled optogenetic control of neural circuits.